The present invention relates to the manufacture of laminated cores for dynamoelectric machines and, more particularly, to the removal of burrs and slivers from laminated cores by means of an electrochemical process.
Laminated magnetic cores are typically fabricated from sheet metal by a stamping process. The individual laminations are then stacked together to form a generally cylindrical assembly. The stack of laminations are then die cast, riveted, welded, clamped or otherwise restrained in order to maintain the geometric integrity of the assembly. When deviations in the geometric size of the laminated assembly due to fabrication variances are intolerable, the outer surfaces of the laminated core can be machined to correct size discrepancies. This machining operation is often used to remove enough material from the rotor to create the desired air gap between the rotor and stator of a dynamoelectric machine. When the rotor and stator laminations are punched from the same sheet metal stock, the rotor laminations are often punched from the central piece which remains after the annular stator lamination is produced. Therefore, in these cases the machining operation is necessary to create an air gap where, otherwise, one would not normally exist. Also, even when the stacked core is not machined, each lamination can have small burrs at its edges as a result of the punching operation itself.
The machining operation, which can either be turning, milling, broaching or grinding, unfortunately creates a multitude of burrs and slivers by smearing the edges of the individual laminations into adjacent laminations. This smearing creates electrical shorts between adjacent laminations and partially defeats the original purpose of the laminated structure which is to minimize the interlaminar conductivity. This smearing deleteriously affects the efficiency of the dynamoelectric machine in which the laminated component is to be incorporated. The original purpose of using individual laminations is to reduce eddy current losses in the dynamoelectric machine and the smearing between adjacent laminations creates eddy current paths which increase interlaminar conductivity and, therefore, core losses.
The above mentioned burrs and slivers can be reduced by buffing, tumbling, shot blasting, sanding or filing the laminated surface following the machining operation. These operations are tedious and costly and tend to be less than perfectly effective. Some means for totally removing the burrs, slivers and smears from the machined surfaces of laminated cores is required because, in electrical apparatus, it is necessary to employ a magnetic core and the electrical efficiency of this magnetic core is dependent upon the amount of electrical insulation between each adjacent lamination so that short circuits are not present between the laminations when the magnetic core is subjected to alternating flux. Under the influence of a magnetic field, each lamination generates a minute electrical potential which could cause a large loss of electrical energy in the form of heat, if it is short circuited to adjacent laminations. Following the machining operation, the burrs and slivers are located almost exclusively at the cut edges of the metal laminations and it is at these locations where short circuits between adjacent laminations can take place because of the presence of these slivers and burrs.
U.S. Pat. No. 2,590,927, which was issued to Brandt et al. on Apr. 1, 1952, describes an electrolytic method for removing the burrs from the cut edges of laminated cores. The method described in the Brandt patent utilizes phosphoric acid in conjunction with an electrical current to render the phosphoric acid active for the intended purpose of removing the above mentioned metallic slivers and burrs. By electrically connecting the laminated core to the positive terminal of a direct current source and disposing a negatively connected cathode proximate the machined surface, an electrical current can be passed through the electrolyte between the machined surface and the cathode. Under the influence of the electrical current, both burrs and slivers are etched away from the machined surface when subjected to the phosphoric acid solution.
The use of an electrical current in combination with a phosphoric acid solution is an improvement over prior methods which utilize acid alone in the absence of an electric current. U.S. Pat. No. 2,293,951 which was issued to Seastone et al. on Aug. 25, 1942 describes such a chemical etching operation which utilizes a solution of nitric, hydrochloric or sulfuric acid without the use of an electric current. An alternative method of treatment is described in U.S. Pat. No. 2,243,578 which was issued to Reardon on May 27, 1941 and which subjects the short circuited laminated core to an acid phosphate treatment.
Electrochemical machining and shaping is also described in U.S. Pat. No. 3,058,895 issued to Williams on Oct. 16, 1962 and in U.S. Pat. No. 3,365,381 issued to Fromson on Jan. 23, 1968. The Williams patent describes the shaping and contouring of electrically conductive and electrochemically erodable workpieces and discloses an apparatus for accomplishing this task. The Fromson patent discloses an apparatus for the electrolytic machining of a workpiece in which an electrode is operated in relation to the workpiece while an electrolyte fluid is passed through a fluid passage.
The use of an acid solution in the electrochemical machining or electrochemical deburring of laminated components introduces serious problems which inhibit the rapid mass production of cores utilizing these methods. When an acid bath is used to electrochemically machine a laminated component, a significant amount of the acid is dispersed over the laminated core and some of the acid solution is distributed between the laminations thereof. Therefore, adequate removal of the acidic solution must be accomplished, following the machining operation, in order that harmful corrosion does not take place within the laminated core following its subsequent assembly into a dynamoelectric machine.
The present invention utilizes a cathode which is disposed a preselected distance from the machine laminated surface and electrically connects the laminated component to the positive terminal of a direct current power source. The present invention utilizes a salt solution, such as sodium nitrate, instead of an acidic solution as an electrolyte between the cathode and the workpiece. The present invention, although preferably utilized with a sodium nitrate solution as the electrolyte, can be alternatively practiced with the use of any conductive electrolyte, such as sodium chloride. Sodium chloride is more corrosive than sodium nitrate and is, therefore, less desirable than sodium nitrate. The electrolyte should be pumped at a flow rate which will prevent a temperature rise above approximately 140.degree. F. in order to prevent boiling. This relatively high flow rate will also diminish boundary layer effects, as will be discussed below, and provide a continuous liquid path across the electrolyte-filled gap.
The cathode of the present invention is equipped with means for delivering a sufficient quantity of electrolyte to the region between the machined surface of the laminated core and the most proximate surface of the cathode. In the preferred embodiment of the present invention, the electrolyte solution is pumped through the cathode and toward the machined laminated surface. The gap between the cathode and the laminated surface is determined by a number of mutually dependent parameters. If the gap is too small, electrical arcing can occur between the cathode and the laminated workpiece and, conversely, if the gap is too large, the effective resistance between the cathode and workpiece can be increased to a point where more power must be used to provide the current through the electrolyte interface. It has been found that a gap of approximately 0.010 inch avoids arcing and is compatible with normally available pump pressures and electrical power supply capabilities.
It has also been discovered that, by using a constant current electrical supply, the temperature in the region of the gap becomes significantly less critical than it would be with constant voltage power supplies. This is due to the fact that the constant current power supply automatically adjusts for rising temperatures or varying salt concentrations by appropriately altering its voltage and thus obviates the need for complex temperature controls or a means for holding salt concentrations with close tolerances. This characteristic greatly simplifies the required control and monitoring systems for electrochemical machining operations.
The present invention, by using a salt such as sodium nitrate, avoids the corrosive characteristics normally associated with acid solutions and reduces the potential hazards which are normally associated with the use of acid solutions such as the possibility of acid splahes and the inhalation of acidic fumes. Another significant advantage of using a salt instead of an acid solution is that the salt solution is essentially nonconsumable and requires only the occasional addition of water to the electrolyte supply during the electrochemical deburring operation. Acid solutions, on the other hand, must be constantly replenished because of their chemical nature and the fact that acids are depleted during electrochemical machining or deburring operations.